Learn Turkish: Sol yoldan gidersen siyah kedi görürsün. - If you take the left road, you see a black cat.

It comes from the verb gitmek (to go). First, we add the aorist suffix -er to the root git, then the conditional suffix -se (if), and finally the personal ending -n (you). Together, git + er + se + n makes gidersen.

Why does the 't' in "git" change to a 'd' in "gidersen"?

This is a standard consonant mutation. When the verb root git (go) takes a suffix starting with a vowel, like the aorist suffix -er, the hard t softens to a d.

Why is "yol" (road) in the ablative case as "yoldan" (from the road)?

In English we say 'take the road' or 'go down the road', but in Turkish, when you describe the route you travel along or through, you use the ablative case -dan or -den (from/via). So yoldan gitmek means to go via the road.

Why doesn't it say "siyah bir kedi" for 'a black cat'?

Because this is a general statement. In Turkish, when talking about things generally rather than one specific item, you can often drop the word bir (a/an). The sentence sounds like a general rule or a superstition.